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Summarizing from dev posts on the Steam forums since it's worth knowing:

The game was written for DirectX 9 and it's preferable to use DX9 (set in the launcher options) unless the game refuses to work in that mode. DirectX 11 mode was only included for use as a troubleshooting option with certain graphics card configurations that wouldn't work at all under DX9.

You *can* use DX11 regardless, but the game doesn't use any of its features, is likely to be less stable, and may even take a framerate hit when running through it.
*Raising from the dead with an olli olli oxenfree*

Thanks for this post. I thought the game ran kind of weirdly on DX11; now I know why.